The invention relates to an apparatus and a method for the feedback control of a grid scanner in ion beam therapy, according to the preamble of claims 1 and 4.
The feedback control apparatus of that kind has, at least, the following devices:
scanner magnet current supply devices for ion beam scanner magnets that deflect horizontally and vertically with respect to the middle of the ion beam, the supply devices being controlled by control and read-out modules for the scanner magnets,
a location-sensitive detector for location measurement, which is controlled by means of a control and read-out module, and
a sequence control device, which controls the activation and read-out sequence among the devices of the apparatus.
A grid scanner of that kind is known from European Patent Application EP 98 117 256.2. That publication describes a grid scanning method with feedback control of intensity. Despite considerable variations in the intensity of the therapy beam, which consists of ions and varies in intensity by a factor of 30 between a maximum value and the average value, that method allows the specifications of an irradiation plan to be applied so precisely that the dose distribution resulting from the entire irradiation differs from the planned dose distribution by less than 5% on average. Feedback control of the intensity accordingly results in the fact that, despite the considerable variations in the intensity of an ion beam, it is possible to adhere very precisely to the total beam dose per beam position.
It is, however, problematic to achieve implementation of a geometrically exact application of the dose distribution, because not only the intensity of the therapy beam but also the beam position of the focussed therapy beam varies significantly during application of the beam. There is currently no complete and efficient solution to that problem. A huge amount of work is currently being expended on measuring those variations in position and drawing up correction tables for each conceivable accelerator and high-energy beam guidance settingxe2x80x94there are, for example, 255 energy stages each having 7 focussing stages and 15 intensity steps that have to be taken into account, so that about 25,000 combinations have to be measured for each beam position and correction tables have to be drawn up for corresponding therapy devices. Such correction tables can then be used to produce the control data for the system; but even that amount of work leads to a positive result only if the differences in the beam position are reproducible for each beam position, which cannot, however, generally be assumed to be the case.
The invention is therefore based on the problem of dramatically reducing the correction work and of significantly increasing the geometric precision.
An apparatus according to the invention for the feedback control of a grid scanner in ion beam therapy has, at least, the following devices:
scanner magnet current supply devices for ion beam scanner magnets that deflect horizontally and vertically with respect to the middle of the ion beam, the supply devices being controlled by control and read-out modules for the scanner magnets,
a location-sensitive detector, which for location measurement is controlled by means of a control and read-out module,
a sequence control device, which controls the activation and read-out sequence among the devices of the apparatus, wherein the apparatus also has in the sequence control device a circuit arrangement having a feedback loop between the control and read-out modules for the scanner magnets and the control and read-out module of the location-sensitive detector and wherein, in circuitry and sequence, the control and read-out modules for the scanner magnets and the control and read-out module of the location-sensitive detector in the sequence control device are technically so arranged that the control and read-out modules for the scanner magnets are arranged serially after the control and read-out module of the location-sensitive detector.
Apart from the quality of dose application, this apparatus also improves patient throughput and, as a result, the economical nature of the apparatus, because the increase in geometric precision reduces the number of interruptions in irradiation owing to interlocks of the location measurement system. Moreover, this solution benefits not only grid scan systems in fixed beam guides having any desired angle but also and especially the grid scan technique in combination with a rotatable beam guide (a gantry), wherein an increase in positional errors can be expected, because such gantry systems are extremely heavy and therefore have a tendency towards mechanical deformation of the beam guide.
Furthermore, the ion optics are very sensitive with respect to variations in position. Homogeneity of the magnetic field, especially in the final deflecting dipole magnet, is extremely difficult to produce, so that the present apparatus according to the invention for the feedback control of a grid scanner brings considerable advantages overall for such gantry systems, because the ion therapy beam in an irradiation position can be readjusted and realigned in accordance with an irradiation plan of the apparatus according to the invention.
In a preferred embodiment of the invention, a multi-wire proportional chamber is used as the location-sensitive detector. Such a detector has the advantage that, on the one hand, the actual state of the beam position can be accurately determined in its location co-ordinates and, on the other hand, as a result of the fact that the location-sensitive detector is coupled into a feedback control loop, the irradiation location can be matched to the irradiation plan and exact concordance between the actual value and desired value can be achieved with respect to the location of the irradiation.
For the purpose of simultaneous feedback control of intensity, the apparatus can preferably have at least one ionisation chamber, which co-operates with a control and read-out module. Preferably, that control and read-out module of the ionisation chamber is, within the sequence control, technically arranged, in circuitry and sequence, before the control and read-out module of the location-sensitive detector. That advantageously results in the fact that first of all the irradiation dose per beam position is monitored and adhered to by controlling the intensity and then, independently of the intensity control, precise positioning of the ion beam can additionally be feedback-controlled with the aid of the location-sensitive detector.
For the method for the feedback control of a grid scanner in ion beam therapy, the grid scanner has the following devices:
scanner magnet current supply devices for ion beam scanner magnets that deflect horizontally and vertically with respect to the middle of the ion beam, the supply devices being controlled by control and read-out modules for the scanner magnets,
a location-sensitive detector for location measurement, which is controlled by means of a control and read-out module, and
a sequence control device, which controls the activation and read-out sequence among the devices of the apparatus,
wherein the following method steps are performed:
comparison of information, deposited in the location measurement control and read-out module of a supervisory control system and relating to the desired position of the beam plan, with the actual measured position of the beam position from a location-sensitive detector, in real time,
determination of a correction value for the scanner magnet supply devices of the grid scanner, and
setting the correction value for the horizontal and vertical magnet supply devices of the grid scanner and realignment of the beam position.
An advantage of the method according to the invention lies in the fact that it is possible to dramatically reduce the measurement of a large number of system settings at the accelerator and/or the beam guide within the context of quality assurance and preparation of the irradiation unit for patient irradiation procedures, by using the location information from the position-sensitive detector before the patient for realignment of the beam position in real time during beam use. Moreover, as a result of this invention, the demands on reproducibility of the beam position are significantly relaxed for all high-energy radiation settings and the geometric precision of dose application improved.
For realignment of the beam position, the information deposited in the location measurement control and read-out module of the supervisory control system and relating to the desired position from the beam plan is compared in real time with the actual measured position from the location-sensitive detector and a correction value for the magnet supply devices of the grid scanner is determined and set. That correction can be made from measurement cycle to measurement cycle of the location measurement system, for example within 150 xcexcs, or alternatively from one beam position in the irradiation plan to the next beam position. Within the real-time control of the system, a series of control and read-out modules are connected to one another by way of interfaces. Of relevance to the invention, however, are the two control and read-out modules that control and read out the location measurement detector and the two grid scanner magnet current supply devices.
For each measurement cycle, the real-time software in the control and read-out module of the location-sensitive detector calculates the actual value of the beam position from the detector raw data and sends that information to the control and read-out modules of the scanner magnets by way of the data connection. For each feedback control cycle, the real-time software in the control and read-out module of the scanner magnets compares the desired position and the actual position and calculates current-correction values for the horizontal and vertical magnet current supply device of the grid scanner and then sets the corrected current values, which result in improved magnetic field settings in the scanner magnets, as a result of which the beam position is improved.
In a preferred embodiment, beam realignment is carried out using damping that can be set by means of the real-time software in the control and read-out modules for the control magnets. As a result thereof, feedback control oscillations are advantageously avoided and carried-over errors are reduced.
In a further preferred embodiment, upper thresholds are fixed in order to limit location correction so that major errors in beam position settings are avoided for reasons of safety. Should such an upper threshold value be exceeded, rapid switching-off of the beam is preferably triggered by the control and trigger module of the location-sensitive detector in real time and, as a result, a chain of switch-off commands is triggered for the various accelerator and beam guidance components.
Further advantages, features and possible applications of the invention are described below in greater detail with reference to exemplary embodiments.